3) Parcels: Everything else that is 13 oz or less (the max weight for First Class)

The post office adds: Flats/Large Envelopes must be flexible. Even if packaged in something that is defined as a Flat (see definition above), the envelope must bend naturally (under its own weight) to be defined as a Flat.

ShipRush for Endicia only processes labels and postage for shipments that include Delivery Confirmation (DC). The post office recently reminded shippers that Flats are not eligible for DC.

So while many shippers have been happily shipping Flats with DC, and all has been well, this is not in conformance with protocol, and will soon no longer be available.